Heretofore, the butadiene-based polymer such as poly-butadiene or the like has been known as a rubber having excellent thermal and mechanical properties, and is widely used in various fields. Particularly, the butadiene-based polymer being high in the cis-1,4 bond content in butadiene monomer unit is excellent in the mechanical properties, so that the production technique of the butadiene-based polymer having a high cis-1,4 bond content is strenuously studied and developed. For example, a composite catalyst system composed mainly of a compound of a transition metal such as nickel, cobalt, titanium or the like is known as a catalyst for the production of the butadiene-based polymer, and some systems are industrially used as a polymerization catalyst for butadiene.
Also, a composite catalyst system comprising a rare earth metal compound and an organometallic compound of Groups I-III is developed as a catalyst capable of producing the butadiene-based polymer with a high cis-1,4 bond content and having an excellent polymerization activity. For example, it is known that when using a catalyst system comprising a neodymium compound and, methylaluminoxane the polymerization can be conducted in a high reactivity and a conjugated diene polymer having a narrow molecular weight distribution is obtained. However, the polymer obtained by this method is not sufficiently high in the cis-1,4 bond content and also a vinyl bond content is not sufficiently low, so that the properties are still insufficient.
JP-A-2001-48940 discloses that the butadiene-based polymer having a very high cis-1,4 bond content is obtained by polymerizing butadiene in a catalyst system comprising a rare earth metal compound, an organometallic compound of Groups I-III and an ionic compound consisting of a non-coordinate anion and cation. In this case, however, there are problems that a special compound such as Nd(OCOCCl3)3 or the like is used as a rare earth metal compound and this compound has a low polymerization activity. Also, since NMR is utilized as a measurement of the micro-structure in JP-A-2001-48940, the error of the vinyl bond content is particularly large and further the value of the vinyl bond content in the resulting butadiene polymer is large, so that the rubber composition containing such a butadiene-based polymer is insufficient in the improvement of the properties as compared with the rubber composition containing the conventional butadiene polymer. Furthermore, the butadiene-based polymer obtained by this method tends to widen the molecular weight distribution as the vinyl bond content becomes low, so that there can not be obtained butadiene-based polymers in which the vinyl bond content is sufficiently low and the molecular weight distribution is within a specified range.
Also, JP-A-2002-241420 discloses that the butadiene-based polymer having a very high cis-1,4 bond content is obtained by polymerizing butadiene in a catalyst system comprising a rare earth metal salt consisting of a rare earth metal, a free halogen atom component and a halogen atom containing component and aluminoxane. In this case, however, there are problems that a special catalyst such as bis(trichloroacetic acid)(versatic acid) neodymium salt or the like is used and the polymerization activity of such a neodymium salt is low and the industrial productivity is low. Furthermore, JP-A-2002-241420 does not examine the vinyl bond content of the polymer at all.
In Aida, Hou, Nishiura, Doi, Kurakado, Horiuchi and Wakatsuki, Macromol. Rapid Commun. 2003, vol. 24, pp. 179-184, is disclosed that the polybutadiene-based polymer having a very high cis-1,4 bond content is obtained by polymerizing butadiene in a catalyst system comprising a metallocene complex of a gadolinium compound. In this case, however, there are problems that the molecular weight distribution of the polymer having a high cis-1,4 bond content is very low of not more than 1.5 and the rubber composition containing such a polymer is low in the workability and difficult in the milling and can not develop the excellent properties. Also, the butadiene-based polymer having a very high cis-1,4 bond content obtained by this method is too high in the molecular weight and has a problem that the workability is very poor in the usual rubber working machine. Further, means for controlling the molecular weight is not disclosed in the above Aida et al report, so that the practicality is low.